In the construction of many buildings, a drop ceiling is spaced below a structural floor panel that is constructed of concrete, for example. Light fixtures as well as other items appear below the drop ceiling The space between the ceiling and the structural floor from which it is suspended serves as a return-air plenum for elements of heating and cooling systems, as well as a convenient location for the installation of communication cables including data and signal cables for use in telephone, computer, control, alarm, and related systems. It is not uncommon for these plenums to be continuous throughout the length and width of each floor. Also, the space under a raised floor in a computer room is considered a plenum if it is connected to a duct or to a plenum.
As a general rule, the National Electrical Code (NEC) requires that cables in plenums be enclosed in metal conduits. However, the NEC permits certain exception to this requirement provided that such cables are tested and approved by an independent testing agent such as the Underwriters Laboratories (UL) as having suitably low flame spread and smoke-producing characteristics.
The flame spread and smoke production of cable are measured using UL 910, Standard Test Method for Fire and Smoke characteristics of Electrical and Optical-Fiber Cables used in Air-Handling Spaces. The UL 910 test, also known as the NFPA 262 test, is considered to be the toughest performance test. Hirshler, in "Analysis of and Potential Correlations Between Fire Tests for Electrical Cables, and How to Use This Information for Fire Hazard Assessment", Fire Technology, Vol. 33, No. 4 (1977) describes various tests used on electrical cables to assess fire hazard, and is hereby incorporated by reference for testing information.
The Steiner tunnel test is quite severe, because it combines a high heat flux with a long duration. Currently, thermoplastic materials such as highly filled polyvinyl chloride (PVC) and fluoropolymers, such as FEP, pass the Steiner tunnel test. However, fluoropolymer materials are somewhat difficult to process. Also, some of the fluorine-containing materials have relatively high dielectric constant which makes them unattractive for communication media.
Further, PVC and fluoropolymers such as FEP are halogenated material. There has been a desire to overcome some problems which exist with respect to the use of halogenated materials such as fluoropolymers and polyvinyl chloride (PVC). These materials exhibit undesired levels of corrosion in fires. If a fluoropolymer is used, hydrogen fluoride forms under the influence of heat, causing corrosion. For PVC, hydrogen chloride is formed.
Silicones, especially non-halogenated polydimethylsiloxanes, are known for fire and heat resistance. Silicones burned in a cone calorimeter have been shown to have a comparatively low peak heat release rate, total heat released, average carbon monoxide production rate, and average smoke production rate as compared with organic compound. See, for example Fu-Yu Hshieh and Robert R. Buch, "Controlled-atmosphere Cone Calorimeter, Intermediate-scale Calorimeter, and Cone Corrosimeter Studies of Silicones, Proc. Int. Conf Fire Safety, Vol. 23, p213-239 (1997). Silicone rubber compositions as jacketing for wire and cable applications are also known, and are generally less expensive than fluoropolymers. However silicone rubber formulations have not previously passed the UL 910 test, and are not currently qualified for plenum use. Silicones are not, for instance, currently listed in the UL 1581 specification for cabling materials. This is believed to be due, in part to poor char formation upon exposure to high heat flux rates. The surface of many silicone rubber compositions used for cable coating crumbles when burned, exposing unburned polymer and substrate, allowing the flame to propagate. Because of these characteristics, no silicones are currently qualified for use in coating plenum cable. The ability to use a non-halogenated polydimethylsiloxane composition as a plenum cable coating would offer a less expensive, non-corrosive material than fluoropolymers.
Recently Sawada, in Japanese patent application 9-55125 described a fireproof electric cable having excellent fire resistance. A fireproof layer of 0.1-1.0 mm in thickness is formed by extruding a coating composition that is 200-500 weight parts of four inorganic fillers to 100 weight parts of hot-vulcanized silicone rubber. The four fillers are glass powder, alumina, wollastonite, and mica. Sawada teaches that all four fillers must be present for sufficient fire resistance. The silicone rubber used by Sawada is hot-vulcanized, and may also contain a crosslinking agent consisting of an organic peroxide, but no information on the chemistry of the silicone is specified. The glass powder consists of particles having a diameter of 50-300 .mu.m and the glass content is 50 to 125 wet parts per 100 weight parts of hot-vulcanized silicone rubber. The alumina which is used consists of particles having a diameter of 50-200 .mu.m, and the content of the alumina is within a range of from 50 to 125 weight parts per 100 weight parts of hot-vulcanized silicone rubber. Wollastonite is used in the form of needle crystals of anhydrous calcium silicate having a particle size of 100-300 .mu.m. The content of the wollastonite is 50 to 125 weight parts per 100 weight parts of the hot-vulcanized silicone rubber. Mica is preferably in the form of flakes with an aspect ratio of about 30-70 ad a particle size of 100-300 .mu.m. The content of mica is 50-125 weight parts per 100 weight parts of the hot-vulcanized silicone rubber. The wire coating of Sawada was tested according to the Fire Services and Fire Resistance Test Act. The test was conducted following a flame curve with a temperature of 840.degree. C. during a 30 min burning process